Postback is triggered only when a user clicks 'OK' button.
Clicking 'Cancel' or outside the control collapses the drop down and restores the check box selection prior to opening the control.
It has same events (in fact only one is of interest - On Selected Index Changed) and members (except those relevant to new rendering and client side behavior).The page contains a drop down check boxes for selecting years and a panel bellow which displays checked items whenever a postback from the control occurs (code for other controls from the picture was cut above): From the example we see that a control event is assigned and the page load handler performs data binding for the control.Whenever a user clicks 'OK' button in the drop down list a postback goes to server and panel in the bottom of the page is filled with all checked items. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. I reloaded a dropdownlist in the pageload, and so the framework lost control of the items (the new ones) in the ddl, and the one i clicked (which didn't exist anymore) in a page.